There has been known a fixing apparatus of the film heating type, which is mountable in an electrophotographic copying machine, an electrophotographic printer, and the like. A fixing apparatus of this type is made up of a heater, a fixation film, a pressure roller, etc. The heater has a ceramic substrate and a heat generating resistor formed on the substrate. The fixation film is placed in contact with the heater. The pressure roller is pressed against the heater, with the placement of the fixation film between itself and heater, forming thereby a nip. A sheet of recording medium on which an unfixed toner image is present is conveyed through the nip of the fixing apparatus while remaining sandwiched by the fixation film and pressure roller, whereby the toner image on the sheet of recording medium becomes fixed to the sheet of recording medium.
A fixing apparatus such as the above described one which employs a heater has a power supply circuit for supplying the heater of the fixing apparatus with electric power. Thus, if the power supply circuit becomes abnormal in operation, it sometimes suffers from the so-called “heater cracking attributable to runaway power supply circuit”), that is, the phenomenon that the heater substrate (which hereafter may be referred to simply as substrate) cracks due to malfunction of power supply circuit for heater). Thus, it is desired that a fixing device of the above-described type is designed so that it can prevent its heater substrate from cracking even if its power supplying circuit for the heater malfunctions. More concretely, if a triac, a relay, and/or the like, which is a part of the above-mentioned power supply circuit malfunctions, the power supply circuit sometimes fails to control its primary current, allowing thereby the primary current to be supplied to the heater. In such a case, the heater abnormally increases in temperature, subjecting thereby its substrate to thermal stress. If this thermal stress is large, the heater substrate sometimes cracks, making the heater unusable. Further, as the heater excessively increases in temperature, a heater holder which holds the heater may melt, which in turn may subject the heater to mechanical stress large enough to cause the substrate to crack. As the substrate of the heater cracks, the heater becomes useless.
One of the methods for preventing a fixing device of the above described type from suffering from the “heater cracking attributable to runaway power supply circuit”, is to design a fixing device so that its thermal fuse, thermal switch, and/or the like component interrupts the primary current before the heater substrate is made to crack by the thermal and/or mechanical stress caused by the abnormal temperature increase of the heater, which is attributable to the flowing of the primary current of the power supply circuit into the heater. In the case of this method, it is required that the heater substrate can withstand the thermal and/or mechanical stress longer than the length of time it takes for a current interrupting member such as the thermal fuse, a thermal switch, and/or the like to react.
There is disclosed in Japanese Laid-open Patent Application 2007-121955 a technology which keeps the heater substrate as uniform as possible in temperature in order to extend the length of time it takes for the heater to crack after the power supply circuit goes out of control. More concretely, according to this patent application, a heat radiating member, which is proportional in thermal capacity to the amount of heat generation of the heat generating member on the “front” surface of the substrate, is attached to a specific portion of the back surface of the heater substrate, more specifically, the portion of the back surface of the heater substrate, which corresponds in position to the portion of the heater, which is higher in the amount of heat generation than the rest, in order to keep the heater substrate as uniform in temperature as possible.
However, the examination of a fixing device similar to the one disclosed in the abovementioned patent application revealed that it is likely that as its heater went out of control, cracking occurs to the portion of the substrate, which is in contact with a current interrupting member such as a fuse.
One of the causes for the above described problem is as follows: The current interrupting member is relatively large in thermal capacity. Therefore, the portion of the substrate, which is in contact with the current interrupting member, is robbed of heater by the current interrupting member, and therefore, reduces in temperature quicker than the rest of the substrate. Consequently, the substrate becomes nonuniform in temperature, which in turn is likely to subject the substrate to thermal stress. Further, because the current interrupting member is in contact with the substrate, the substrate is also subject to the mechanical stress attributable to the current interrupting member (substrate is pressed by current interrupting member), adding to the amount of the stress to which the substrate is subjected.
There are some cases in which a current interrupting member is attached to the substrate with the placement of a spacer made of resin, between the current interrupting member and substrate. In such cases, the spacer made of resin may melt, and therefore, the current interrupting member may come into contact with the substrate, which in turn may cause the substrate to crack as described above. Further, there are some cases in which a current interrupting member is improperly attached to the substrate due to the errors which might occur during the assembly of the heater. More concretely, if a current interrupting member is fixed to the heater substrate in such a manner that it is tilted relative to the substrate, it may come into contact with the substrate. That is, if a current interrupting member such as the thermal switch, and/or the like is tilted relative to the substrate, the end of the hard metallic member of the current interrupting member, may contact the substrate, causing the mechanical stress attributable to the current interrupting member to concentrate on the point of contact between the current interrupting member and substrate, subjecting therefore the substrate to a very large amount of force. Thus, it is more likely for the substrate to crack at the point of the substrate, which corresponds in position to the current interrupting member, as the power supply circuit goes out of control.
Further, in the case of some fixing apparatuses of the film heating type, their heater holder is provided with through hole(s), and the current interrupting member is placed in the through hole of the heater holder in such a manner that it is placed in contact with the heater substrate. In other words, the hole has to be made through the heater holder for the attachment of the current interrupting member to the heater substrate. Thus, the portions of the heater holder, which have the hole for the current interrupting member, is less in mechanical strength. While the heater is normal in operation, the heater holder can satisfactorily hold the current interrupting member. However, as the heater goes out of control and causes the heater holder to soften (or melt), the portion of the heater holder, which has the hole for the current interrupting member, fails to support the current interrupting member, allowing the current interrupting member to sink into the heater holder, allowing thereby the current interrupting member to directly come into contact with the heater substrate. In other words, the heater (substrate) is subjected to an additional stress, making it likely for the heater (substrate) to crack.
In recent years, it has come to be required that an electrophotographic copying machine, an electrophotographic printer, and the like are reduced in the FPOT (First Page Out Time; length of time required to output first print), and increased in PPM (Pages Per Minutes; number of prints which can be output per minute). In order to meet such a requirement, it is necessary to supply the heater of a fixing apparatus with a substantially larger amount of electric power than that by which a conventional fixing apparatus is supplied with electric power. Because of the circumstance described above, there is desired a fixing apparatus which can more effectively prevent the problem that as its power supply circuit goes out of control, its heater cracks, than a fixing apparatus in accordance with the prior art.